In many asphaltic compositions it is particularly desirable to have a high degree of flexibility combined with toughness and durability. High ductility is especially desirable for meeting the specifications demanded in industrial asphalts used in materials such as roofing shingles, built-up roofs, canal linings, pipe coatings, etc. Additionally, because of the temperature stress to which these asphalts may be subjected, it is desirable to have an asphalt which can withstand low temperatures over time without developing significant brittleness and not flow at high temperatures. The parameters of these conditions may also be expressed as the well known physical properties of asphalt: viscosity, penetration and softening point.
The Penetration Index (PI), an expression of the relationship between penetration and softening point, is one preferred measure of the properties of industrial asphalts. Industrial asphalts usually have a PI greater than +2; as distinguished from paving asphalts, for example, having a PI of 0 or less. See "Properties of Asphaltic Bitumen", J. Ph. Pfeiffer, ed. (1950), page 166 et seq. Industrial asphalts also normally have softening points in the range of 220.degree.-240.degree. F. and penetration of 15-20 at 77.degree. F., dmm.
One well known may to increase the Penetration Index is by modification of the asphalt by air-blowing. Air-blowing is a process by which a low softening point asphalt is converted to a more desirable asphalt by the forced introduction of air at temperatures ranging 400.degree. to 500.degree. F. The air-blowing process is also ordinarily facilitated by the addition of an asphalt catalyst which reduces the time required to bring asphalt to the desired specification and improves the softening point/penetration relationship of the asphalt product.
One way of increasing the toughness or flexibility of paving asphalts (versus industrial asphalts) is the addition of polymers or elastomers to the asphalt blend. One widely-known polymer, ethylene/vinyl-acetate copolymer (EVA), increases the ductility and toughness of paving asphalts, and therefore might have been thought to do likewise with industrial asphalts. However, it has been found that in the simple blending of EVA with already air-blown, high softening point industrial asphalt, the blend was proved incompatible. Specifically, softening points and viscosities were so greatly increased as to render the asphalt unsuitable for the desired purpose.
The present invention, however, has discovered a method for avoiding this incompatibility problem using a particular asphalt/EVA blend and air-blowing methods.